background image

AIM

10/12/17

7−3−1

Wake Turbulence

Section 3. Wake Turbulence

7−3−1. General

a. Every aircraft generates a wake while in flight.

Initially, when pilots encountered this wake in flight,

the disturbance was attributed to “prop wash.” It is

known, however, that this disturbance is caused by a

pair of counter−rotating vortices trailing from the

wing tips. The vortices from larger aircraft pose

problems to encountering aircraft. For instance, the

wake of these aircraft can impose rolling moments

exceeding the roll−control authority of the encounter-

ing aircraft. Further, turbulence generated within the

vortices can damage aircraft components and

equipment if encountered at close range. The pilot

must learn to envision the location of the vortex wake

generated by larger (transport category) aircraft and

adjust the flight path accordingly.

b. During ground operations and during takeoff,

jet engine blast (thrust stream turbulence) can cause

damage and upsets if encountered at close range.

Exhaust velocity versus distance studies at various

thrust levels have shown a need for light aircraft to

maintain an adequate separation behind large turbojet

aircraft. Pilots of larger aircraft should be particularly

careful to consider the effects of their “jet blast” on

other aircraft, vehicles, and maintenance equipment

during ground operations.

7−3−2. Vortex Generation

Lift is generated by the creation of a pressure

differential over the wing surface. The lowest

pressure occurs over the upper wing surface and the

highest pressure under the wing. This pressure

differential triggers the roll up of the airflow aft of the

wing resulting in swirling air masses trailing

downstream of the wing tips. After the roll up is

completed, the wake consists of two counter−rotating

cylindrical vortices. (See FIG 7−3−1.) Most of the

energy is within a few feet of the center of each

vortex, but pilots should avoid a region within about

100 feet of the vortex core.

FIG 7−3−1

Wake Vortex Generation

7−3−3. Vortex Strength

a. The strength of the vortex is governed by the

weight, speed, and shape of the wing of the generating

aircraft. The vortex characteristics of any given

aircraft can also be changed by extension of flaps or

other wing configuring devices as well as by change

in speed. However, as the basic factor is weight, the

vortex strength increases proportionately. Peak

vortex tangential speeds exceeding 300 feet per

second have been recorded. The greatest vortex

strength occurs when the generating aircraft is

HEAVY, CLEAN, and SLOW.

b. Induced Roll

1. In rare instances a wake encounter could

cause inflight structural damage of catastrophic

proportions. However, the usual hazard is associated

with induced rolling moments which can exceed the

roll−control authority of the encountering aircraft. In

flight experiments, aircraft have been intentionally

flown directly up trailing vortex cores of larger

aircraft. It was shown that the capability of an aircraft

to counteract the roll imposed by the wake vortex

primarily depends on the wingspan and counter−

control responsiveness of the encountering aircraft.